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87 votes

How many times do you have to circle the Earth to break orbit?

This depends on how much thrust you have available. With enough thrust, you don't need to be in Earth orbit at all: you can launch straight into an escape trajectory. New Horizons did this, more or ...
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78 votes
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Shouldn't space junk fall back to Earth on its own? How long will take for a ~1 cm piece of junk in LEO to fall back to Earth on its own for example?

It depends on the altitude. Here is a chart from ESA and UNOOSA. Basically, anything under 500 km will fall relatively quickly, maybe 25 years. Everything under 800 km should fall within a century or ...
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70 votes

Where does space begin on planets without atmospheres?

Yet surely nobody would classify Neil Armstrong's lunar bunny-hops as suborbital spaceflights. Why not? There's no essential difference between a high-eccentricity trajectory with an apolune of 1 ...
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66 votes
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What is the farthest artificial satellite in orbit in our solar system?

The term for orbits in our solar system around the Sun is Heliocentric. Closed Heliocentric Orbit The solar observation probe Ulysses is the furthest artificial satellite around the sun. It's in a ...
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66 votes
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Why can't you travel to Mars in a straight line?

Only answering your first question here, and in qualitative terms: You can't travel to Mars in a straight line for the same reasons you can't throw a ball in a straight line: gravity. If you wanted to ...
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64 votes

Why aren't the 'bedrooms' aboard the ISS also the escape pods?

Primarily, because without a lot of extra equipment, they could at best be space coffins for the astronauts. First, they share life support with the rest of the station. Air circulated through them ...
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61 votes

Why doesn't ISS pass over the polar regions?

The decision to orbit (and assemble) the International Space Station (ISS) in 51.65° prograde Low Earth Orbit (LEO) was driven by its accessibility and utility. You see, inclination changes are ...
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60 votes

Why are all space initiatives designed to get to space overnight when in Nature all extreme feats are achieved in a gradual process

Imagine you have a very heavy book and a bookcase, and your goal is to put the book on the top shelf of the bookcase. How much time would you spend doing that? Maybe five seconds, maybe fifteen. Would ...
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59 votes
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Why do artificial satellites need orbit correction, but natural ones don't?

The instability in orbits of our artificial satellites come from a few basic causes: Atmospheric drag and solar wind effects The Earth isn't a perfect uniform sphere but is slightly lumpy, which ...
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55 votes
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What does the Sun-Earth-Moon system look like from the Sun-Earth L-2 point?

I treated this as a problem of geometry and came up with this: The sun is the large yellow disk. The earth is the largest black disk, obscuring most of the sun The left-hand dark-grey disk is the ...
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51 votes

Why are all trajectories in space a conic sections?

Not all trajectories in space are conic sections, only those that are a two body problem. One planet in orbit around a star is a two body problem. Only two body problems are solved by a conic section. ...
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49 votes
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Determining orbital position at a future point in time

Statement of the Problem The problem you want to solve is called the Kepler problem. In your formulation of the problem, you're starting out with the Cartesian orbital state vectors (also called ...
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  • 2,490
46 votes

How many times do you have to circle the Earth to break orbit?

Zero See at: https://en.wikipedia.org/wiki/Escape_velocity for theory. Once you build enough velocity to surpass gravitational attraction, you will leave planetary orbit. A spacecraft simply ...
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  • 499
46 votes
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Do you need 0 km/s velocity to crash into the sun?

Wouldn't i inevitably spiral to sun surface even if i was faster than 0km/s ? No. On reasonable timescales, an orbit will have a fixed distance of closest approach, called "periapsis." (These ...
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  • 4,517
45 votes
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Are all satellites of all planets in the same plane?

Most large regular satellites orbit in the equatorial plane of their planet. Every planet spins, and thus has an equatorial bulge caused by centrifugal forces that over time aligns the satellites' ...
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44 votes

Escaping moons conflict with what I understand of gravity

A very good question! The reason is essentially to do with tides. And a slightly over-simplified summary is: If the moon orbits more slowly than the rotation of the parent body (as our Moon does, 12 ...
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42 votes

Why do artificial satellites need orbit correction, but natural ones don't?

Essentially, this is a result of observational bias. A natural satellite will only orbit a parent for extended time periods precisely because the orbit it is in is stable †. The plain truth of the ...
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41 votes
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Why use a retrograde orbit?

Retrograde orbits have multiple use-cases. First of all you should note that "retrograde" doesn't mean 180° inclination - everything > 90° is considered retrograde. This places all sun-synchronous ...
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  • 6,770
41 votes

If the Earth spun clockwise, how would that affect Space Exploration?

Primarily, locations of spaceports would change. California, not Florida would host the NASA's main launch site. Russia would be in slightly better position, able to send rockets over the Black Sea, ...
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37 votes
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Requirements to orbit Pluto

This mission study came up with a 900 kg nuclear-electric-propulsion spacecraft launched on an Ariane V with a C3 of 100 km2/s2 and a Jupiter gravity assist along the way. 1.05 kW electrical power at ...
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  • 57.6k
37 votes

Do simulator games use a realistic trajectory to get into orbit?

I understand the reasons behind each of this manouveurs, however I'm wondering if this is how real rockets get into orbit. Cutting off the engine and letting the rocket loose vertical speed looks ...
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36 votes
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How does a Spacecraft change its orbit?

To answer your title question: By using its engines. However you seems to be quite puzzled by the fact that velocity of an object can decrease and increase over the course of an orbit. If the orbit ...
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35 votes

Where does space begin on planets without atmospheres?

Ultimately, "the edge of space" is an agreed upon convention. In other words it is essentially arbitrary, something which only humans even care about (well maybe space aliens too <grin>). Yes, ...
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34 votes

Do simulator games use a realistic trajectory to get into orbit?

Physics In regards to the physics, KSP is fairly realistic, other than it not modeling n-body physics (which isn't really relevant in scope of orbiting Earth/Kerbin). In regards to the engineering, ...
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  • 441
33 votes
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How did the USSR track Gagarin's Vostok-1 orbital flight? Was tracking capability an issue in the choice of orbit?

The Answer is on a page by Sven Grahn. No ocean going tracking ships were used. Only ground stations on the territory of the USSR. In the other answers some russian sources about the use of tracking ...
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32 votes

Why did the X-15 not achieve orbit?

You're confusing units. The maximum speed of the X-15 was 7274 km/h, or about 2 km/s. Orbital speed is around 8 km/s. The X-15 didn't carry enough fuel to reach orbital speed.
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32 votes
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How is the altitude of a satellite defined, given that the Earth is not spherical?

update: 6378.137 km is what I use now. By convention the altitude of a spacecraft is the distance to the center of the Earth minus roughly 6378 kilometers, or some reference radius that is ...
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31 votes
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Is this really Rosetta's orbit around 67P?

This was one of the questions just now during the Rosetta press briefing. This video was shown during the presentation: The triangular trajectory are hyperbolic ...
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30 votes
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Why are the GPS constellation satellites in such a high orbit?

The main reason they are in such a high orbit is to allow for more of the Earth to be visible at any one time. In order to have a reasonable amount of the Earth visible, you have to be high up. A ...
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  • 118k
30 votes
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This orbit looks wrong near a Lagrange point. Is it?

Is the orbit shown in the graphic wrong, or is my understanding of orbital mechanics lacking, having only been influenced by KSP? It's not an either-or question. The graphic is "wrong" from the ...
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